Piston type fluid motor having separately fed piston shoes



April 8, 1969 c. A. KUBILOS 3,437,015

PISTON TYPE FLUID MOTOR HAVING SEPARATELY FED PISTON SHOES Filed Oct. 30, 1967 Sheet of 5 INVENTOR. CHARLES A. KUBILOS BY WOOD HERRON 8| EVANS ATTORNEYS April 1969 c. A. KUBILOS 3,437,015

PISTON TYPE FLUID MOTOR HAVING SEPARATELY FED PISTON SHOES Filed Oct. 30, 1967 Sheet 2 of 3 INVENTOR. CHARLES A. .KUBILOS BY WOOD,HERRON a EVANS ATTORNEYS Api'il 8, 1969 c. A. KUBILOS 3,437,015

PISTON TYPE FLUID MOTOR HAVING SEPARATELY FED PISTON SHOES Sheet ofS Filed oct. 30, 1967 WITH SEPARATE SHOE FEED// IOO .01 0 e55 FLOW (PERCENT OF MAXIMUM-LOG SCALE FLOW (PERCENT OF MAXIMUM-LOG SCALE 6 INVENTOR. CHARLES A. KUBILOS wooo, HERRON a EVANS ATTORNEYS 3,437,015 PHSTON TYPE FLUID MOTOR HAVING SEPARATELY FED PISTON SHOES Charles A. Kuhilos, Oxnard, Califi, assignor to Abex Corporation, New York, N.Y., a corporation of Deiaware Filed Sci. 30, 1967, Ser. No. 679,015 Int. Cl. F0119 13/04, 31/10; F0411 1/22 U.S. Cl. 9257 11 Claims ABSTRACT F THE DISCLOSURE A fluid motor of the piston type in which the individual pistons have shoes mounted to them for engaging the cam plate, wherein pressure fluid from a common pressure source is supplied through the barrel to an intermediate chamber in each cylinder from which it is permitted to seep at a restricted rate along the piston wall into a groove or chamber in the piston wall adjacent the active or inner end of the piston, from which it is supplied through the piston to the shoe. The low speed performance of the motor is improved by reduction of leakage to tank and reduction of friction due to piston side loads.

This invention relates to improved means for supplying pressure fluid to the piston shoes of a fluid motor of the piston type.

The conventional piston type fluid motor includes a barrel which is journalled for rotation in the pump casing and connected to drive a shaft. Several piston cylinders or bores are formed in the barrel, and typically although not always these bores are spaced equally and have axes which are parallel to the axis of the barrel. Each bore has a piston which is reciprocable in it. At its outer end each piston is provided with an element generally called a shoe that facially engages and slides over a cam or swash plate. The plane of the cam plate surface angularly intersects the shaft axis.

Pressure fluid supplied from a source into the pressure or control chambers in the bores at the inner ends of the pistons urges the pistons into contact with the swash plate and tends to extend the pistons which are innermost in their bores, thereby causing the barrel to be driven rotationally.

In order to lubricate the sliding movement of the piston shoes over the cam surface and to provide a degree of hydrostatic pressure balance on the pistons, it is known to supply pressure fluid from the control chamber at the inner end of each cylinder bore, to the region between the face of the piston shoe and the cam surface. One prior means of providing such pressure to the piston shoes is shown in Neff et al. patent No. 3,049,940, issued August 21, 1962, wherein pressure is supplied from the control chamber through an axial bore in the piston and through the shoe to a recess formed in the shoe adjacent the cam surface.

While the high speed operation of such motors is good, test data indicates the relatively poor low speed operation of axial piston motors having piston shoes supplied with pressure fluid directly through the piston from the pressure chamber. In particular, low speed performance of such motors (below roughly 100 rpm.) has been poorer than would be desired because of excessive leakage of fluid from the pressure chamber at the inner end of the piston through the piston and out from under the piston shoe. During low speed operation, the shoes intermittently may tend to lift off the cam surface, and when that occurs a direct path is opened through which fluid is discharged from the piston pressure chamber into the pump casing, which is essentially at tank pressure.

States Patent 0 Another respect in which the low speed performance of conventional axial piston motors has been relatively poor, is in the rather high frictional forces which act on the pistons due to side loading. The reaction force of the cam plate on the piston shoe is not directed along the axis of the piston, but includes a transverse component which tends to cock or tip the piston in its bore, so that there is metal-to-metal contact between the piston and barrel at the inner end of the piston, and where the piston extends out of the barrel.

This invention is directed to overcoming these disadvantages of prior piston motors, and is predicated upon the discovery of means whereby both leakage and friction are reduced so that low speed operation is significantly improved.

In broad terms, the present invention contemplates means for supplyling pressure to the individual piston shoes which does not draw fluid from the piston control chambers, but rather which supplies pressure fluid from a common source to a zone in the cylinder bore from which the fluid is permitted to flow inwardly through a restricted passage between the piston Wall and the cylinder wall, to a groove or recess formed in the piston side wall near to but not opening to the control chamber, and from which a passageway leads internally through the piston into the shoe.

By this invention pressure is fed to the shoes not through the inner end of the piston, but rather from an entirely separate pressure port in the piston cylinder. The pressure fluid from the port in each bore seeps back along the piston wall to the piston groove, from which it is fed the shoe balance pad. Since the shoe balance pad ordinarily has essentially the same area as the piston, the system is stable only when the piston groove pressure is equal to the control pressure behind the piston; therefore, there is no significant pressure differential between the control chamber at the inner end of the piston, and the piston groove. Hence the discharge of fluid from the control chamber via the shoe is effectively reduced.

But in addition to advantages in respect to reduction of leakage, another important advantage occurs by reduction of piston side wall friction. It has been found that, in motors provided with the structure of this invention, the pressure fluid forces on the side wall of the piston tend to center the piston axially if it is cocked, thereby reduciug friction.

The invention can best be further described by reference to the accompanying drawings, in which:

FIGURE 1 is a diagrammatic illustration of a hydraulic circuit including an axial piston motor, shown in longitudinal section, provided with piston shoe pressure supply means in accordance with a preferred embodiment of the invention;

FIGURE 2 is an enlarged section of a piston and shoe, showing the piston cocked in its bore;

FIGURES 3 and 4 are charts showing the relative magnitudes or profiles of the pressures acting on the upper and lower sides, respectively, of the cooked piston shown in FIGURE 2;

FIGURE 5 is a chart showing the relationship between flow and operating speed of a motor of the type shown in FIGURE 1; and

FIGURE 6 is a chart showing the relationship of flow and the pressure differential between the inlet and outlet ports of the motor.

Referring to the drawings in more detail, the hydraulic circuit shown in FIGURE 1 for purposes of illustration includes a pressure source designated generally by 10 which supplies pressure fluid through an adjustable pressure control valve 11 to a hydraulic motor 12 embodying a preferred form of the structure of this invention.

More particularly, the pressure supply source includes a fluid reservoir or tank and a pump which has an inlet line that receives fluid from tank :15. An electric motor or other prime mover 17 drives pump 16. Excess pressure in the pump outlet or discharge line 18 is spilled to tank 15 by a relief valve 19.

The particular valve 11 comprises no part of the invention and may be conventional inasmuch as its function is to adjustably control the pressure and direction of application of pressure fluid from pump .16 to the fluid motor 12. The particular valve 11 shown for purposes of illustration is of the electrically controlled two stage jet tube type valve shown in R. D. Atchley Patent No. 2,884,907, issued may 5, 1959, to which reference may be had for a more complete description. It should be understood that the particular construction of valve 11 is not critical to an understanding of the present motor, and valve 11 can even be a manually operated valve. Valve 11 may be reversible, as shown, to reverse the direction of rotation of motor 12.

The pump outlet or pressure line 18 is connected to the inlet ports 20 and 21 of valve 11, and the valve work ports 22 and 23 are connected respectively to the ports 24 and 25 of hydraulic motor 12. That motor port 24 or 25 to which pressure fluid is delivered at a particular setting of valve 11 will comprise the inlet port, and the other port 25 or 24 will comprise the outlet port. Valve 11 also has a tank port 26 from which a line returns to tank 15. Motor 12 has a casing or tank 27 from which a line returns to tank 15.

Motor 12 includes a casing comprised of a generally cylindrical center body portion 35, a port block 36 which is secured and sealed by suitable means at one end of body 35, and an end or head plate 37 mounted at the other end of body portion 35. The motors main ports 24 and 25 are formed in port block 36, and the tank port 27 is formed in body portion 35.

End plate 37 and body 35 cooperate to define an internal chamber 38. An annular area 39 is formed within body 35 for receiving a roller bearing 41. This bearing rotatably journals the cylinder barrel 42, which drives a shaft 43. The barrel is loosely connected with the shaft in any suitable manner, such as by means of splines 44. It will be understood that shaft 43 is connected to operate driven means (not shown). The barrel member 42 is spring urged axially of shaft 43 into abutting relation with the port surfaces 46 by a coil spring 47. The port surfaces 46 may be provided on a separate port plate, or may as shown he defined by lands on the inner end of port block 36. The coil spring 47 is compressed between the end of the splined portion of shaft 43 and the inner end of an axial opening 48 formed in barrel 42.

Cylinder barrel 42 is provided with a plurality, for example nine, of longitudinally extending cylindrical bores 49. These bores are preferably equally spaced about the circumference of a circle and communicate with corresponding openings 51 extending from the face of the cylinder barrel which abuts the port surfaces 46. The bores 51 are disposed to be brought into alternate registration with arcuate ports 52 and 53 formed in port block 36, which communicate respectively with ports 24 and 25 thereof. Each bore 49 receives a piston 55 for a reciprocating movement within it. Pressure fluid from the inlet port of the port block is admitted into the cylinder bore to cause the piston to be extended when the corresponding barrel port 51 is in registry with that port 52 and 53 of the port block which comprises the inlet port, and fluid is expelled from that port. 51 which is in registry with the outlet port in the port block.

Reciprocating movements of pistons 55 are effected by means of a cam assembly 56 mounted upon end plate 37. This cam assembly includes an angulated swash or cam plate 57 carried by plate 37. The angulated cam surface 59 of the swash plate is engaged by hearing shoes 60 mounted by ball and socket joints to the respective ends of the pistons 55. Specifically, each of the pistons 55 is provided with a spherical head portion 61. This head is received and held within a socket formed in the respective shoe '69. Each of the bearing shoes 68 passes through an opening formed in a retainer or hold-down plate 62. This plate 62 abuts flanges or shoulders 63 formed on the shoes 60 and thereby maintains the shoes in engagement with swash plate 57. The retainer plate 62 is rotatably journalled in a bearing 64 carried by a sleeve 67 which is threaded into an opening in swash plate 57. This opening is formed on angle such that the axis of the opening is perpendicular to the cam surface 59 of the cam plate. Alternatively, the motor may be provided with means for varying the angulation of the cam plate, thereby varying the volume or displacement of the pistons, in known manner.

Shaft 43, to which barrel 42 is connected, is rotatably journalled in suitable bearings (not shown) carried by end plate 37.

It is to be understood that the motor environment thus far described is merely exemplary and it is contemplated that the present separate pressure supply means can also be applied to other piston type fluid motors, including pneumatic as well as hydraulic motors.

A shoe recess 68 is formed within and bounded by a circumferential land 69 which engages and rides upon the cam surface 59. Pressure fluid is supplied into the recess 68 through an axial bore 71 in the shoe, which communicates at all times with an axial bore 72 formed in the respective piston. The 'bore 72 in the piston communicates, through an angularly extending bore 73, with a recess or circumferential groove 74 formed in the piston side wall adjacent the inner or active end 75 of the piston.

The portion of the barrel bore 49 behind (i.e. toward the port block 36) the inner end 75 of the piston constitutes a control chamber 77 pressure in which tends to move the piston outwardly (to the right in FIGURE 1). Groove 74 is separated from the end 75 of the piston, and hence from chamber 77, by a land 78 defined on the piston side wall.

Port block 36 of motor 12 has a separate pressure port 80, to which the pressure line 18 of pump 16 is connected. Pressure fluid from port 80 is constantly supplied through a passageway 81 in the port block and a rotating seal at 82 into a short central bore 83 in barrel 42. Branch passages 84 communicate between bore 83 and ports or chambers in the side walls of the respective bores 49. In the embodiment shown the latter ports are comprised by a circumferential groove 85 formed intermediately in each cylinder bore 49. Groove 85 is preferably positioned so that it is never directly aligned with the piston groove 74, nor does it open directly to the tank chamber 38 when the piston is at its innermost position. Alternatively, rather than a circumferential groove in the cylinder bore, a recess can be provided in the piston or line 84 can simply enter bore 49 directly.

OPERATION Pressure fluid is continuously supplied from the pressure source 10 to passage 81 in the port block, and through the rotating seal at 82 into each of the branch passages 84 in the barrel and into the respective cylinder bore grooves 85. The pressure in groove 85 is reflected entirely around the piston, and thus of itself imparts no side-wise force to the piston. The pressure fluid in groove 85 flows at a restricted rate between the piston side wall and bore 49, into groove 74 at the inner end of the piston, from which it is applied through bores 73, 72, and 71 into the shoe recess 68.

It is unnecessary to provide any extra or special clearance or tolerance for fluid communication between the groove 85 formed in the cylinder bore, and the groove 74 formed in the piston. Ordinary manufacturing tolerances will permit suflicient flow, and the clearance between the piston side wall and the bore may be essentially constant over the length of the piston.

It can be seen that there is no direct communication between the control chamber 77 and the shoe recess, so that if the shoe is intermittently lifted off the cam surface 59 there can be no unrestricted escape of fluid from the shoe. The escape of pressure fluid from groove 85 to recess 68 is restricted at all times by the very narrow annulus between them. The pressure in shoe recess 68 tends to equalize with the pressure in chamber 77 to balance the piston, and the pressure in piston groove 74 at all times equals the pressure in shoe recess 68. Hence, a substantial pressure balance exists between the pressure in control chamber 77 and groove 74, and the leakage of fluid between them is substantially zero. The spacing of groove 85 from the outer end of the barrel restricts leakage past the piston to tank. However, by reason of the reciprocation of the piston in its bore the path between bore groove 85 and the piston groove 74 is relatively short at one point in the cycle of piston movement, and the supply of pressure fluid to recess 68 is at a maximum rate once each revolution of the barrel.

In addition to reducing fluid leakage, the separate shoe pressure supply system described also establishes fluid pressure force vectors which act on the piston in a manner tending to recenter it when it is cocked or tipped in its cylinder bore. This is best explained by reference to FIG- URES 2, 3, and 4. As shown in exaggerated form in FIGURE 2, when a piston is tilted so that it is in metal-to-metal contact with the bore at one point a at the end of the bore and at an opposite point b on its inner end, different pressure distributions are established between the piston side wall and the cylinder bore on opposite sides of the piston.

The pressure drops off more rapidly from a pressure source in the direction of piston divergence from the cylinder wall than it does in the direction of piston convergence with the wall. Thus, as measured along the upper side of piston 55, pressure qualitatively follows the general form of the curve in FIGURE 3. The pressure in chamber 77 is at a value designated as P and from point b rises rather abruptly at piston groove 74, then rises at a decreasing rate to the maximum pressure supplied by the pump P in the vicinity of groove 85. From this level P the pressure drops off along at a gradually decreasing rate, to essentially tank pressure where the piston extends beyond the barrel. Along the diametrically opposite line on the piston surface, the pressure follows the profile shown in FIGURE 4 but acts in the opposite direction. From the control pressure P, pressure rises at a gradually increasing rate to the maximum P at groove 85. From groove 85 the pressure drops at an increasing rate to tank pressure at point a at the end of the bore 49. The pressure distribution of course varies around the piston, but there is a resultant force passing perpendicularly through the axis of the piston in the plane of the sheet. The shaded areas in FIGURES 3 and 4 indicate unbalanced or net pressures acting in each direction. A force F can be visualized as acting upwardly on the piston ahead (i.e. toward the shoe) of groove 85, and tends to increase the clearance at point a. A force F can be visualized as acting downwardly on the piston inwardly (i.e. toward chamber 77) of groove 85, and tends to increase the clearance at point b. As a result of these forces the piston tends to be more nearly centered so that frictional forces are reduced.

Actual test data confirms the improvement in low speed motor operation resulting from the provision of separate piston feed means in accordance with this invention. FIGURE 5 of the drawings compares the variation of motor speed as a function of flow through the motor, for a motor having conventional piston shoe feed means directly from the control chamber, with the results obtained in an otherwise similar motor provided with separate piston feed means in accordance with this invention. It can be seen that at low flow rates, below about 1% of maximum, the conventional motor does not rotate at all and virtually all of the flow is wasted as leakage. In contrast, in a motor provided with separately fed piston shoes, rotation begins at a very low flow and thereafter rises essentially in direct proportion to the flow.

Moreover, the differential pressure "between the motor ports 24 and 25 is very different as between a conventional motor and a motor in accordance with this invention. As shown in FIGURE 6, in the conventional motor which was tested the high friction along the piston walls resulted in a high pressure differential at about 1% flow. As the pump began to rotate the differential dropped rapidly to about p.s.i., and then rose gradually as flow increased. On the other hand, in a motor with separately fed piston shoes, the reduction in internal friction was such that the pressure differential remained much more nearly uniform at about 100 p.s.i., over the same region at which the conventional motor did not operate at all or exhibited an undesirably high pressure differential.

It should be understood that the invention has been described primarily in relation to a specific hydraulic axial piston motor shown in the drawings, but that the separate shoe means of the invention can be employed in other piston type fluid motors which, for example, are not of the axial piston type or which have a different form of shoes or other means for engaging the cam surface.

While the present separate pressure feed means are most useful in improving the operation of motors, they may be also incorporated on pumps of the piston type where, for example, the pump is convertible for use as a motor.

Having described my invention, I claim:

1. In a fluid motor of the type having a rotatable barrel with a plurality of pistons reciprocable in bores therein, each piston having a shoe mounted to it for engaging a cam surface,

the improvement comprising,

a port entering each bore through the side thereof,

each said port being closed at all times by the respective piston,

a passage through said barrel for supplying pressure fluid from an external source to each of said ports,

a recess in the side wall of each piston,

each piston having a passage within it for supplying pressure fluid from said recess to the shoe mounted thereon,

the clearance between said bore and said piston side wall providing a restricted path for the flow of fluid from said port to said recess.

2. The improvement of claim 1 wherein said port is a circumferential groove in said bore.

3. The improvement of claim 1 wherein said recess is a circumferential groove in said piston side Wall.

4. The improvement of claim 3 wherein said groove is adjacent to but spaced from the inner end of said piston by a land on the side wall of said piston.

5. The improvement of claim 1 wherein said port and recess are defined by a circumferential groove on said bore and piston side wall respectively and are so positioned that said recess is spaced inwardly of said port throughout the range of operating movement of said piston.

6. The improvement of claim 1 wherein said groove is spaced inwardly of said port at all times.

7. The improvement of claim 1 wherein the clearance for flow of fluid from said port to said recess between said piston side wall and said bore is essentially the same clearance as exists over the remainder of the length of said piston.

8. The improvement of claim 1 wherein pressure fluid is supplied into said passage in said barrel through a rotating seal lying on the axis of said barrel.

9. The improvement of claim 8 wherein the casing of said motor has a conduit for supplying pressure fluid from a pump to said rotating seal.

10. The improvement of claim 1 wherein said motor is an axial piston hydraulic motor and wherein said shoes are mounted to said pistons through ball and socket joints.

11. An axial piston hydraulic motor having a rotatable barrel with a plurality of pistons reciprocable in bores in said barrel, each piston having a shoe mounted to it for engaging a swash plate,

a first groove in each bore intermediate the ends thereof,

a passage in said barrel for supplying pressure fluid from an external source to each said groove, a second groove formed on the side of each piston adjacent to but spaced from the inner end thereof,

and a passage within each piston for supplying fluid from said second groove to the shoe mounted to said piston.

References Cited UNITED STATES PATENTS Phifer 9271 Davis.

Michell 92-57 Strimel 92165 X Pedersen et a1. 103162 Thoma 9216O X Firth et a1 103162 Thoma 92158 X Hulman et a1. 103-162 MARTIN P. SCHWADRQN, Primary Examiner.

l5 IRWIN C. COHEN, Assistant Examiner.

US. Cl. X.R.

U.S. DEPARTMENT OF COMMERCE PATENT OFFICE Washington, 0.6. 20231 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,437,015 April 8, 196$ 7 Charles A. Kubilos It is certified that error appears in the above identified patent and that said Letters Patent are hereby corrected as shown below:

Column 7, line 16, after "thereof," insert the clearar between each said bore and the side wall of each said piston providing a restricted path for the flow of fluid from said first groove to said second groove,

Signed and sealed this 7th day of April 1970.

(SEAL) Attest:

Edward M. Fletcher, Jr. WILLIAM E. SCHUYLER, J]

Attesting Officer Commissioner of Patents 

